here are the object i see being able to use:
1) arrows (you can easily change endpoints locations+styles, color, transparency)
2) stamps (clipart), should rotate, scale, move, set transparency; includes cursors, sparkles, etc.
3) simple text boxes (should be able to choose color, shadow, justification, roundborder/noborder,transparency)
4) clipart text callouts (like baloons for text)
5) highlight boxes and underlines
all objects should be movable, deletable, etc.
clipart should be in standard format for easy adding new stuff.